MIPS: ralink: mt7628a.dtsi: Add pinmux DT node

This patch adds the pinmux DT node using the generic "pinctrl-single"
pinmux driver. For this the system-controller register area needs to be
changed to not overlap with the pinmux registers.

This patch is based on work done by John Crispin.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Harvey Hunt <harveyhuntnexus@gmail.com>
Cc: John Crispin <john@phrozen.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org